South Jersey Skies: Parties in the dark

Late December has been a time for celebrations throughout history and around the world.

Keth JohnsonIn China, there is Dong Zhi. For the Jews, it's Hanukkah. The Incas celebrated Inti Raymi; the Persians, Yalda; the Romans, Saturnalia. Christians have made their Christmas celebration universal.

(Of course, Christians did not intend Christmas to commemorate anything to do with the Sun. But historians have suggested that they chose the solstice time because their people were accustomed to observing the older pagan festivals. At some times in history, Christians may even have employed the observance as protective camouflage. In fact, the date of the birth of Jesus is not known.)

Even Facebookers have their Kwansolhaneidmas. I guess some holidays are more serious than others.

Why this time of year? After all, this is a time of maximum darkness and considerable cold, surely not a good time for making merry. But the important word is “maximum.” Future days will be brighter, so let's party!

Diagram showing the tilt of the Earth at the four seasons. The rightmost Earth is at showing winter in the Northern Hemisphere. The diagram is not to scale.The underlying reason for all this comes from the relationship of the Earth to the Sun. The line through the North and South Poles, the Earth's rotation axis, does not sit “straight up and down” in space, perpendicular to the plane of the Earth's orbit around the Sun. Rather, it's tipped over by 23.5 degrees.

You can see a depiction of this in most Earth globes: they're tipped at this angle. You might have thought this was to make them easier to use, but that's not the reason.

The key fact for us today: the rotation axis does not change its orientation, its direction in space. The north end of that line always points up almost perfectly to the North Star. The Earth is a massive gyroscope, with a lot of spin momentum, and it would be very difficult to flip it around very much in space, as many people envision it doing.

Rather, the change in the seasons comes about only because the Earth is at different positions in its orbit, relative to the Sun, during the year (the seasons have little to do with the changing distance of the Sun from Earth).

At one point in its orbit, in late June, our Northern Hemisphere is “leaning toward the Sun,” maximally exposed to its warming rays, so it tends to be warm there.

Six months later, in late December, the Northern Hemisphere is pointing away from the Sun, so it's cold. The rotational axis hasn't shifted direction, but the Earth is on the other side of the Sun now.

You can observe the evidence for this easily, but you must devote some time to the project. Mark and record the altitude of the Sun in degrees above the southern horizon at local noon every clear day for a year (I told you it would take some time! I suggest you draft your children to help.)

You might do this by measuring the length of the shadow of a straight pole stuck vertically in the ground. You'll find the length of the shadow reaches a maximum, meaning the altitude of the noon Sun is at its lowest, around December 21. On that day, we're tilted most directly away from the Sun. Astronomers call this the Winter Solstice; the rest of us call it the first day of winter, though that's not an official title, believe it or not — some cultures have even labeled the time “Midwinter.” There are fewer hours of daylight now than any other day: maximum dark!

(Technically, the solstice is the direction in space, a point on your star chart in Sagittarius, occupied by the Sun at its maximum southerly position. But most of us think of the solstice as a time, not a direction. Please don't tell the Solstice Police on me.)

On this day, the up-and-down motion of the noonday Sun ceases. The Sun at noon seems to hang at the same altitude for several days, standing still (as far as up and down motion is concerned, anyway.) Hence the name: “Sol” meaning “Sun,” “-stice” from the Latin for “still.”

You can also observe interesting changes at sunset (and sunrise, but let's focus on sunset.) Sit on a boulder that's not likely to move for a year, in the middle of a large field. Have your assistant, perhaps a handy graduate student, carry a smaller rock, perhaps painted bright red, some distance away from you (the farther she goes, the more accurate this will be.) Direct her to set it down in line with the setting Sun.

Perform this experiment for a year. You'll find the sunset marker rocks will reach a point farthest to the left of west on December 21, the same day you see the lowest noontime Sun. After this day, the rocks will start marching to the right. To conserve your research budget, I recommend doing both of these experiments in the same year.

The powerful astronomer-priests who performed these measurements in past centuries knew that when the lowest-noonday-Sun and farthest-left-sunset points were observed, they had reached the darkest day. After this, the Sun would day-by-day return to its higher position in the sky; the air would eventually warm; it would soon be time to plant the new crops. Of course we should celebrate the Return of the Sun! We should also pay a substantial fee to those astronomers who had such wondrous knowledge of the world. Ah, the good old days!

There are many other fascinating customs and celebrations connected with the winter sky. You can experience them in “Season of Light,” the annual holiday star show at Edelman Planetarium, which also includes a presentation on the Star of Bethlehem. This show has been so popular that an expanded schedule will be offered. The show will play at 7 and 8:30 p.m. on Saturday nights, and 3 p.m. on Sundays, the first three weekends of December.

Also playing is “Holiday Magic,” a laser light show featuring songs of the season by Mannheim Steamroller, Andy Williams, Bing Crosby, the Spice Girls and many more. This show plays on Saturdays at 7:45 p.m. and Sundays at 3:45 p.m. the same days as “Season of Light.” Admission fees and show descriptions can be found at www.rowan.edu/planetarium/. We hope you can pay us a holiday visit!

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Keith Johnson will celebrate the return of the Sun at 1:47 p.m. EST, Dec. 21, in his role as director of Edelman Planetarium at Rowan University. He's glad to answer questions by e-mail at johnsonk@rowan.edu or phone at (856) 256-4389.